Project Summary When HIV-1 is transmitted from one individual to another, a major bottleneck takes place and only one or a few viral variants establishes infection in the new individual. The study of these special transmitted/founder (T/F) viruses is important because these viruses seed new infections. Since T/F viruses are the most important targets of prevention based strategies, a deeper understanding the phenotypic properties that correlate with successful transmission is crucial. Several phenotypic features of the Env glycoprotein clearly differentiate T/F HIV-1 from those that arise during chronic infection or after lab adaptation: T/F Envs are exclusively CCR5 tropic, they have fewer glycosylation sites and shorter variable loops, and they are more resistant to neutralizing antibodies. In this proposal, I will characterize a new feature of T/F HIV-1 variants which I have discovered: T/F HIV-1 is highly specific for the CD4 receptor of humans, whereas lab adapted HIV-1 is far more promiscuous and can enter cells through the CD4 molecules of all primate species that I have tested. This distinction is important because it further supports the idea that the lab adapted viruses that we have been studying for decades may not truly be representative of viruses that are naturally transmitted during infection. The objectives of this proposal are to study the evolutionary diversity of CD4 to identify receptor requirements that are critical to the successful entry and infection of T/F HIV-1 variants. The specific aims of this research proposal are to: 1) determine the extent to which CD4 imposes selection on lentiviruses during transmission, 2) investigate post- entry interactions between primate CD4 and Nef and Vpu from T/F HIV-1, and 3) identify nonhuman primate individuals with CD4 alleles best suited for studying HIV-1 transmission and early infection. Collectively, this research will provide critical insights into how we tackle modeling T/F HIV-1 infection in nonhuman primates. In addition, it will further our understanding of the biological interactions made between T/F viruses and their hosts, as well as aid in our understanding of how lentiviruses adapted to humans during zoonosis.